# Potentiometer

#### Rockgirl

Aug 26, 2021
22
Hi, I need help. Can I find any potentiometers where the rotation angle is smaller than usual? Usually it's I guess 290 degrees but can somewhere find about 90 degrees? I need a 100k potentiometer that adjusts the 90 degree rotation, I don't know if they are anywhere.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,651
Can you use a 330 kΩ potentiometer and limit the rotation mechanically to 90 °?
I'm not aware of 90 ° potentiometers off the shelf.

#### Rockgirl

Aug 26, 2021
22
Can you use a 330 kΩ potentiometer and limit the rotation mechanically to 90 °?
I'm not aware of 90 ° potentiometers off the shelf.

Oh I'm not sure. Maybe I need to open up more about what this is all about. I bought a bidirectional DC-motor controller. I would like to use a potentiometer on the joystick. But the joystick can't turn it enough. If there was any trick to make this work?
Maybe this brings even more difficulty, but I would supply this controller with either 12vdc or 24vdc. Does it make this more difficult?

I tried to put a picture of the connection but it doesn't work

I hope you could help me with this problem.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,651
I tried to put a picture of the connection but it doesn't work
Resize to < 300 kB.

I bought a bidirectional DC-motor controller.
Which model? Can you provide a link to the technical data / datasheet of the controller?

#### bertus

Moderator
Nov 8, 2019
3,268
Hello,

Why do you need a 90 degrees potmeter?
The circuit uses a standard 100 k potmeter:

Bertus

#### Rockgirl

Aug 26, 2021
22
Hello,

Why do you need a 90 degrees potmeter?
The circuit uses a standard 100 k potmeter:
View attachment 52768
Bertus
I already mentioned it above in the post. Because I want use joystick and the trajectory of the joystick is not enough.

#### bertus

Moderator
Nov 8, 2019
3,268
Hello,

You can adjust the range for the circuit with the 100k and 10k resistors on adjusting stack.
You could make the circuit and measure the voltage on pin 3 of IC1a.
Then you can calculate the needed resistors in stead of the 100k and 10k.

Bertus

#### Rockgirl

Aug 26, 2021
22
Hello,

You can adjust the range for the circuit with the 100k and 10k resistors on adjusting stack.
You could make the circuit and measure the voltage on pin 3 of IC1a.
Then you can calculate the needed resistors in stead of the 100k and 10k.

Bertus
Oh I guess I don't know how to do that

Moderator
Nov 8, 2019
3,268

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,651
You're not alone in searching a pot with 90 ° rotation angle. All references I can find mention standard 270 ° pots with mechanically limited rotation angle. The ones that have "90 °" in the title of the sales offer refer to the angle between the pins and the potentiometer axle as 90 °, not the rotation angle.

In your circuit the pot is used as a voltage divider together with the 10 kΩ resistor at the lower end and the 100 kΩ resistor at the upper end. It thus delivers these voltages to pin 3 of IC1a:
0.05 × V+ when in the lower position,
0.52 × V+ when in the upper position
You cannot achieve this exact ratio by simple means. You can approximate it, however:
• Remove the upper 100 kΩ resistor by a short circuit.
• Change the lower 10 kΩ resistor to 5 kΩ (use 2 × 10 kΩ in parallel or use 4.7 kΩ, good enough)
• Change the gain of IC1a from 1×to 2× by adding two resistors in the feedback as shown e.g. here. The value of the 2 additional resistors is not important. both shall have the same vale in the range 1 kΩ to 10 kΩ, whatever you have in your parts bin.
• Mechanically limit the potentiometer to move from the left side (lower pin in the diagram) to left side + 90 °.
Using 4.7 k will give you these divider ratios:
0.1 × V+ in the lower position
0.72 × V+ in the upper position.
You can tweak (fine tune) these values by changing the gain f the opamp IC1a from 2 to another (lower) value. Do this by changing the resistor values in the feedback circuit according to the equations given in the link I providd above.

#### Rockgirl

Aug 26, 2021
22
You're not alone in searching a pot with 90 ° rotation angle. All references I can find mention standard 270 ° pots with mechanically limited rotation angle. The ones that have "90 °" in the title of the sales offer refer to the angle between the pins and the potentiometer axle as 90 °, not the rotation angle.

In your circuit the pot is used as a voltage divider together with the 10 kΩ resistor at the lower end and the 100 kΩ resistor at the upper end. It thus delivers these voltages to pin 3 of IC1a:
0.05 × V+ when in the lower position,
0.52 × V+ when in the upper position
You cannot achieve this exact ratio by simple means. You can approximate it, however:
• Remove the upper 100 kΩ resistor by a short circuit.
• Change the lower 10 kΩ resistor to 5 kΩ (use 2 × 10 kΩ in parallel or use 4.7 kΩ, good enough)
• Change the gain of IC1a from 1×to 2× by adding two resistors in the feedback as shown e.g. here. The value of the 2 additional resistors is not important. both shall have the same vale in the range 1 kΩ to 10 kΩ, whatever you have in your parts bin.
• Mechanically limit the potentiometer to move from the left side (lower pin in the diagram) to left side + 90 °.
Using 4.7 k will give you these divider ratios:
0.1 × V+ in the lower position
0.72 × V+ in the upper position.
You can tweak (fine tune) these values by changing the gain f the opamp IC1a from 2 to another (lower) value. Do this by changing the resistor values in the feedback circuit according to the equations given in the link I providd above.
Hi, I would like to try this. Would it be possible for you to show (draw) how this is done for example in this picture you added?
I’m a girl and this isn’t my strongest skill, not yet at least I'm afraid if I destroy this controller by doing wrong I want to be sure what I do. I know how to solder and I know some basic stuff about electronics but I’m not sure about this.

#### Attachments

• bi_speed_contol.png
267.2 KB · Views: 2

Moderator
Nov 17, 2011
13,651
Here you go:

#### Rockgirl

Aug 26, 2021
22
Okay, I'll try this tomorrow. I still have to ask if I understood correctly, so can I add those new two resistors to something between 1k -10k? Thanks

#### Rockgirl

Aug 26, 2021
22
The solution was found closer than I could imagine. I found an old joystick potentiometer. It looks bigger compared to other potentiometer. I check those multimeter and I get 110k ohms. This potentiometer work those angle which I want. But why do these pots work differently? Are joystick pots some way different?

Another issue related to this bi-directional control. Is it a feature of this control that it does not start running the engine completely at zero? When I rotate the potentimeter the motor starts to buzz at the beginning and then it suddenly starts to rotate. I don’t mean it starts rotating at full, but about 3-4 volts. Why doesn't the motor run in the 1-3v range?
No matter which potentiometer is used for this, the potentiometer will not affect this. I would like this to work in the same way when I give power to the motor power supply unit, when I start from zero and add volts the motor starts a little quietly spinning up to 1 volts.

#### Rockgirl

Aug 26, 2021
22
I have a question about this bidirectional motor controller. Can this be made to work below 5VDC? I have a few of these controllers and some of them won't even work on 5VDC.

#### Bluejets

Oct 5, 2014
6,863
Can this be made to work below 5VDC
I'd imagine the voltage range of the LM324 and the voltage at which the motor will operate would be the limiting factor.
Also remember there is a protection diode in the supply line making any 5v applied drop to around 4.3V if diode is everyday silicon.
There may also be some biasing as well as diffierent mosfet type (logic level) changes required to make the mosfets turn on appropriately.

Might be better to start again from scratch.
Fairly certain if you posted just what it is you are trying to achieve completely, there would be some module or circuit out there to suit.

Last edited:

#### Rockgirl

Aug 26, 2021
22
I'd imagine the voltage range of the LM324 and the voltage at which the motor will operate would be the limiting factor.
Also remember there is a protection diode in the supply line making any 5v applied drop to around 4.3V if diode is everyday silicon.
There may also be some biasing as well as diffierent mosfet type (logic level) changes required to make the mosfets turn on appropriately.

Might be better to start again from scratch.
Fairly certain if you posted just what it is you are trying to achieve completely, there would be some module or circuit out there to suit.
Okay, maybe it's better that I don't use less than 5 voltage, maybe it's better to use 6 voltage at its lowest. There is no other circuit that could affect this controller. These controls are powered by a power supply. I only choose how much the voltage controller goes. I have three options, 5, 12 and 24VDC. But I am going to change 5VDC to 6VDC.

I have more problems. I don't know why but some controller mosfet break. I'm not sure but it seems that it happens at 24 voltage. Or can it be caused when I change 12 to 24 voltage. This switching is done with a relay. I don't really understand why IRFz44n or IRF4905 break. The only thing I have changed from the original controller is the potentiometer. Some have 150k and some 50k ohm. This is because I had to use the joystick. A normal 100k ohm potentiometer won't work because you have to turn it too much.
If anyone knows why mosfet break?

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